You'll arrive on a hunk of island that is actually a pretty large mountain poking out of the water. Land and then climb up the steps until you run across a Veteran. She uses a Skarmory L68 (middle), a Donphan L68 (left), and a Kingdra L68 (right) in a Triple Battle, so watch out!
Just a bit past her is a house where you can heal up at. Talk to the Ace Trainer inside and she'll heal you up to full. The shady guy will also buy Rare Bones for $10,000 apiece, and they can be found — although rarely — being held by wild Crustle.
To the south of the house is a boulder that can be moved with Strength. There's a Black Belt there with a Hariyama L65, a Hitmontop L65, and a Throh L65. Pick up the Zinc to the southwest of him as well as the hidden Rare Candy to the southeast of him.
Now head west of the house, heading down the stairs. There's actually a Hidden Grotto near the base of the stairs (it's the patch of trees at the bottom of the large flight, but right before the small steps).
|Route 18 Hidden Grotto|
|Kingler Lv. 55-60
|Corsola Lv. 55-60
|Chatot Lv. 55-60
|Dragonite Lv. 55-60
Head south until you find a little pond. The rock sitting in the sand in the very southeast corner of that little area has a hidden Pearl String, which can be sold to the ore maniac for an impressive $25,000. Go back north until you find a boulder to the east that can be pushed. Inside of that little pit is a Veteran. He has only a single Pokemon on his team, but it's a mighty Tyranitar L70. Take him out and then pick up the Star Piece he was guarding.
Back at the stairs leading down and the Hidden Grotto, head north and fight the Hiker. He uses a Hippopotas L64, Crustle L64, and a Hippowdon L64. After beating him, head south along the grass until you run into a Veteran. She's got just one Pokemon: a Salamence L70! Just past her is TM19 (Telekinesis).
Return to the Hiker except this time head north, wrapping around the northern part of the island until you find some stairs leading up a cliff. Up there, you'll find Crasher Wake! Yeah, that's right, the Gym Leader from the Sinnoh region. He'll just be singing about himself for awhile before noticing you and talking for a bit. He'll ask if you want to know his technique or something like that. Say yes and he'll head off to Driftveil City to participate in the Pokemon World Tournament, I guess.
Keep going east until you find another Veteran on a bridge. He uses a Staraptor L68, a Krookodile L68, and a Relicanth L68 in a Rotation Battle. Right past him is the Magma Stone, which is a key item that triggers running into the legendary Pokemon Heatran when in the basement area of the Reversal Mountain that Bianca was investigating during your trip there.
Further east and down some stairs, you'll find yourself back at a beach. Pick up the hidden Heart Scale on the beach, then get in the water and take the bottom current to swim your way all the way over to the P2 Lab and the rest of Route 17.